One form of sprung vehicle seat, as disclosed for example in U.S. Pat. No. 4,484,723, comprises a base frame and a seat carrier frame which is displaceable relative thereto, with a fluid spring disposed between the two frames. The seat construction includes a control means for controlling the fluid spring, including a switching means and a sender or actuator which is operable to act on the switching means, to vary the pressure in the fluid spring. Movements in respect of height of the seat carrier frame relative to the base frame are converted into a relative rotary movement between the switching means and the actuator. The switching means for altering the pressure in the fluid spring comprises two stitches and the actuator which operates the switching means is in the form of a control spool. Each of the two stitch devices has two actuating stages wherein during a pivotal movement of the spool the first actuating stage initially acts as an idle stage and a switching operation is produced only in the subsequent second actuating stage. In the decreasing actuation mode, the switch which has been switched on remains in that condition until both actuating stages have been traversed. For that purpose the control spool has two control portions wherein, in both directions of pivotal movement of the spool, it is firstly the first control portion which puts the respective operable switch into the first actuating stage that comes into operation and it is only upon further movement in the same direction that the second control portion which moves the respective switch into the second actuating stage comes into effect.
In that vehicle seat structure, the control arrangement performs only a relatively small angular movement during a movement in respect of height of the seat carrier frame relative to the base frame, so that the relative rotary movement between the switching means and the actuator in the form of the control spool member is relatively small. As a result of that comparatively slight rotary movement, the various switching operations must be carried out within the relatively small angle of pivotal motion of the actuator.
German laid-open application (DE-OS) No. 30 43 177 discloses a sprung vehicle seat in which the seat carrier frame is supported on a base frame by way of guide levers, with an interposed fluid spring. The control means for adjusting the pressure in the fluid spring is associated with one of the two frames. In that vehicle seat design also the control means has a switching means and an actuator for actuating same. The switching means comprises two switch devices and the actuator is in the form of a slide member. The actuator is arranged displaceably on the seat carrier frame and has two flanks or side surfaces which, in the event of heightwise motion of the carrier frame relative to the base frame, come into operative co-operation with one switch device or the other so that the pressure in the fluid spring is either increased or reduced. In that construction, the linear displacement of the actuator, being dependent on the movement in respect of height of the seat carrier frame relative to the base frame, is comparatively slight so that the control arrangement must also be of a precise design so that it can operate with a good degree of accuracy.